This invention relates generally to navigational aids. More particularly, the present invention relates to a method and apparatus for plotting a boat's or a yacht's position relative to a rhumb line when the boat tacks.
During boat racing the racing course is typically delineated by three marks that are located at twin points, preferably on an equilateral triangle, which is oriented relative to the direction of the wind for at least the start of the race. Ideally, the race consists of six legs with some of the legs in the direction of the wind, some of the legs downwind and some of the legs at different angles against the wind.
A windward leg is defined as the leg of the course which lies between two marks located in substantial alignment with the wind direction and which requires that the boat sail against the direction of the wind. No boat can sail directly into the wind, because the sails will not fill as necessary to impart a propelling force to the boat. However, at certain angles relative to the wind the sails become quite effective. Although each boat is somewhat different, in general a boat can sail effectively into the wind on a heading of about 45.degree. relative to the eye of the wind. By thus beating to the wind along a successive series of starboard and port tacks, a boat can arrive at a point directly upward of the starting point.
To change from a starboard to a port tack, or vice versa, is called tacking. During the tacking maneuver, the bow of the boat passes through the eye of the wind while the sail changes its position from one side of the boat to the other. As the bow of the boat heads more and more into the wind, the sails begin to luff and lose their ability to drive the boat. If the boat had sufficient speed at the beginning of the maneuver, and if the maneuver is carried out smoothly, the momentum of the boat will swing the bow through the eye of the wind and beyond until the boat is once more headed in the direction that will permit the wind to fill the sails and impart the necessary driving force to the boat.
Continuing with the explanation of the windward leg, it is deemed appropriate to facilitate an understanding of the present invention, the most direct route is directly into the wind is an imaginary line called a rhumb line. This rhumb line represents the direct line between the start of the windward leg and the end point.
As is often the case, when the boat enters the windward leg, the next mark may not be visible from the start of the windward mark. Accordingly, the captain of the boat must use a chart or a map to plot the boat's course. However, when the wind shifts, the captain must constantly recalculate and then re-plot the boat's course.
Without elaborate facilities on-board to plot the course, it is difficult to determine the most advantageous headings to the windward mark, or the time on which the boat must sail on one tack relative to the other tack. These difficulties are compounded as previously described when one is unable to see the windward mark from the location where the race was started.
Various racing calculators have been constructed to try to alleviate some of the problems previously discussed. Examples of these calculators are disclosed in U.S. Pat. Nos. 4,689,476 and 4,855,577. However, these calculators only determine numbers such as time, distance or headings in which the boat is to travel, but do not show a relative position of the boat with respect to the rhumb line. Further, these calculators may be bulky or cumbersome to use when actually racing.